The present invention concerns vacuum pumps and, in particular, turbo molecular pumps that are used in semiconductor manufacturing processes requiring a vacuum environment with a pressure lower than atmospheric pressure. More specifically, the present invention concerns the use of vibration dampers between the vacuum pump and a vacuum environment, such as a vacuum chamber, in order to isolate the vacuum environment from any vibration generated by the pump.
In semiconductor manufacturing processes, a variety of steps, from layer or film deposition to inspection, are performed in a vacuum environment. However, because the vacuum pump is constructed with extremely tight tolerances extending down to the millimeter range, which enables operation with free molecular flow, the pump can be the source of a significant problem with vibration. This problem is particularly acute with turbo molecular pumps, having a floated rotor and stator construction, where rotational speeds are attained in the range of 50,000 rpm or greater.
The achievement of proper vibration isolation between the pump and the vacuum chamber is particularly important where the semiconductor structure is in the submicron range. The unwanted effects of vibration include errors in line deposition and film formation, and even errors in the inspection and quality assurance process, where extremely high accuracy in comparing patterns on a manufactured substrate against a reference pattern is required, and vibration anomalies may lead to erroneous decisions on product quality.
Such problems arise in inspection systems using scanning electron microscopes (SEM) or comparably sensitive devices, having less than one micron field of view, where inspection of a specimen (typically a wafer) is performed with the generation of an electron beam applied in a specimen chamber that must be maintained in a low pressure and contamination-free environment.
An example of a conventional turbo-molecular pump of the type manufactured by Varian Corp. or Pfiffer Edwards is illustrated in FIG. 1, where the pump 100 has a cylindrical outer body 101. As illustrated in the figure, the pump has a central axis C-C and an inlet port 103 defined by a rim 102 that is adapted to attach directly, or be coupled via a conduit or manifold, to a vacuum chamber (not shown). At an opposite axial end of the cylinder body 101 is an exhaust port 104 to which the contents of the vacuum chamber are exhausted. The pump exhaust port is radially disposed with regard to the central axis C-C and is located on one side of the pump body 101. Preferably, a conduit 105 for electrical, hydraulic, gas purge and cooling hose connections (collectively 130) is also radially disposed. At the same axial end, the bottom of the pump body has a sealing plate 106 that is removable but also serves as a support. The interior of the body 101 defines a chamber containing a rotor 107 that is disposed for rotation along the axis C-C and is supported by magnetic bearings 108 and mechanical bearings 109. The rotor 107 drives rotating blades 110, which are disposed radially with respect to the central axis C-C. Stator blades 111, also disposed radially and interposed between the rotator blades 110, are affixed to a support adjacent to the inner surface of the body 101, in a manner well known in the art. The rotor 107 is supported by a frame 112, and is mounted to the body 101 by vibration damping connectors 113 via arms 114 on the rotor body 112. A motor 115 is operative to drive the rotor 107 at high speed, in the range of approximately 50,000 rpm or higher.
A coupling of the molecular-turbo pump 100 to a vacuum chamber is conventionally implemented with the use of a vibration damper 150, as illustrated in FIG. 2. Elements in FIG. 2 having a reference numeral identical to those in FIG. 1 refer to the same structure and are not further described. The vibration damping mechanism 150 is coupled at one end to the rim 102 of the pump 100 at input 103 via a lower clamp 160 and is coupled at the other end to the inlet port 180 via an upper clamp 170. The clamp 160 fits around the rim 102 and a lower distal end 151A of the vibration damping structure 150 and is secured by a plurality of bolts (unnumbered). At the opposite distal end 151B of the vibration damping structure, clamp 170 serves to couple the vibration damper 150 to the structure of the vacuum chamber inlet port 180 and is similarly secured by a plurality of bolts (unnumbered). The coupling of the turbo molecular vacuum pump 100 to the inlet port 180 via the vibration damper 150 defines a “serial-coupled” damper and vacuum pump arrangement. One or more centering rings 162 (which are conventional and available off the shelf, for example, at www.duniway.com) may be secured by the clamps 160, 170 and sealed by an O-ring 161, as is known in the art.
The vacuum damper 150 comprises a rubberized support 152 that extends between the connector portions 151A and 151B at the opposite distal ends of the damper. The structure is made of a hardened rubber and has coupled to its interior surface a plurality of baffles 153. The vacuum damper 150 is a conventional design that is available off-the-shelf from several vendors.
Although the serial type arrangement illustrated in FIG. 2 eliminates some of the vibration that originates in the pump 100, there continues to remain a problem with residual vibration. As illustrated by U.S. patent Pub. 2001/0012488 to Ohtachi et al, entitled VACUUM PUMP, particularly in FIG. 4 of the Otachi et al publication, a series type connection may be used in which a damper is interposed between an input port of an external container and an outer cylindrical portion of a vacuum pump in order to prevent pump-origin vibration from being propagated to the external container. The damper uses a thin SUS-made cylindrical member bent into a bellow shape, which is coated with a silicon rubber or the like. The damper has a natural frequency of 20 Hz or less. However, the damper requires extra space in the axial direction of about 10 cm, thereby increasing the size, complexity of the structure, and cost of construction, assembly and maintenance. In order to resolve this problem, the Ohtachi et al patent depresses the propagation of vibrations to an external container without the use of a damper, by applying a vibration-absorbing member between a stator portion and a base. Nonetheless, as illustrated in FIG. 5 of the Otachi et al publication, a bellows and extended flange continues to be required. The disadvantage of such a system is that vacuum power is significantly decreased. The additional distance between the pump input port and the input port of the vacuum chamber, as well as the bellows structure itself, reduces the effective speed of the pump. Thus, for a given pumping requirement, a much larger and more expensive pump is required.
The present invention is intended to solve this problem by allowing a direct connection between the pump and a vacuum chamber inlet port, thereby increasing conductance with accompanying reduction in resistance, while providing vibration damping with a damper assembled in a nested fashion about the pump. The nested arrangement may be considered a parallel, rather than serial connection of the damper structure.